This application is based on Japanese Patent Application HEI 11-309851, filed on Oct. 29, 1999, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a semiconductor light reception device and more particularly to a semiconductor-light reception device in which light is incident upon an end face of a light reception layer. A data transmission speed of 40 GHz or higher is required for high-speed optical communications. Attention has been paid to pin type photodiodes as a light reception element capable of high-speed operation.
b) Description of the Related Art
InGaAs is used as a material for detecting light having a wavelength of 1.55 xcexcm used in optical communications. In order to absorb almost 100% of light incident upon an InGaAs layer along its thickness direction, it is desired to set a thickness of the InGaAs layer to 3 xcexcm or thicker. It takes some time for an external circuit to detect carriers generated by light absorption and moved in the InGaAs layer along its thickness direction to the external circuit. If a data transmission speed is low, a propagation time of carriers in the InGaAs layer does not become a critical issue. However, at a data transmission speed over 30 GHz, it becomes difficult to speed up the data transmission speed because the carrier propagation time hinders it.
If light is made incident upon an end face of a thinner InGaAs layer and generated carriers are propagated along a thickness direction of the thinner InGaAs layer, sufficient light can be absorbed and a carrier propagation time can be shortened. The above problem can be solved in this case.
Generally, InGaAs or the like is used as the intrinsic layer (light reception layer) of a pin type photodiode in a band of a wavelength of 1.55 xcexcm, and InP or the like is used as the p-type and n-type layers. An operation speed is limited by a frequency of 1/(2ΠCR) where C is an electrostatic capacitance of a pin type photodiode and R is its load resistance. In order to realize a high-speed operation, it is therefore desired to make the electrostatic capacitance C smaller.
The electrostatic capacitance C can be made small by thickening the light reception layer. However, as the light reception layer is made thick, a carrier propagation time taken for the carriers to move along the thickness direction and reach the interface becomes long. In order to reduce the electrostatic capacitance C without thickening the light reception layer, it is desired to reduce the area of a pin junction.
It is difficult, however, to incorporate a process of forming a photodiode with a pin junction of a small area by cleaving a wafer on the whole surface of which pin junctions are formed. For example, the light reception characteristics of pin type photodiodes become different if cleaving positions vary.
It is an object of the present invention to provide a semiconductor light reception device suitable for a high-speed operation and easy to manufacture.
According to one aspect of the present invention, there is provided a semiconductor light reception device comprising: a substrate having a principal surface; a photo sensor formed in a partial area of the principal surface of the substrate, the photo sensor including a light reception layer parallel to the principal surface, the light reception layer being made of semiconductor and generating carriers in response to received light; a light waveguide formed in a partial area of the principal surface of the substrate, the light waveguide propagating light in a direction parallel to the principal surface and introducing light into the light reception layer; an insulating or high resistance side protective film covering at least a portion of a side face of the photo sensor; and electrodes for flowing current into the light reception layer of the photo sensor in a thickness direction of the light reception layer.
Since the photo sensor and light waveguide are formed on the same substrate, the size of a chip can be maintained large to some degree even if the photo sensor is small. When chips are cut from a wafer, the wafer is cut across the light waveguides and not across the photo sensors. The reappearance of the characteristics of semiconductor light reception devices can be maintained stable.